Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 NERVOUS SYSTEM OF VERMES. 149 Similar nerves are given off from the oesophageal ring to the alimentary canal. The concrescence of two separate structures in the ventral chord of the Gephyrea is the reverse of the permanent separation of the two halves of the ventral chord, which obtains in other divisions of the Annulata. It would not, however, be safe to regard these stages as lower ones, until observation shall have shown, which it has not done yet, whether or no they are preceded by an
Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 NERVOUS SYSTEM OF VERMES. 149 Similar nerves are given off from the oesophageal ring to the alimentary canal. The concrescence of two separate structures in the ventral chord of the Gephyrea is the reverse of the permanent separation of the two halves of the ventral chord, which obtains in other divisions of the Annulata. It would not, however, be safe to regard these stages as lower ones, until observation shall have shown, which it has not done yet, whether or no they are preceded by an earlier stage, like that in the Gephyrea. The connection of two separate ventral chords by means of transverse commissures would be more easily explicable if the ventral chord were previously single. The nervous system of Sagitta has a peculiar character. Lateral commissures from the cerebral ganglion in the head, pass backwards and downwards to the ventral surface of the body, and pass into a large ventral ganglion, which lies just below the integument, aud gives off peripheral nerves to all sides. § 119. A higher grade of differentiation is seen in the nervous system of the Hirudinea and Annelides. The cerebral ganglia are connected by commissures with a ventral chord, and so far these groups re- semble the Gephyrea. In many Annelids the two halves of the ventral chord are homogeneous, and only indicate their metameric character by giving off nerves. In most, however, there are central- form-elements regularly distributed along it. The ventral chord then appears to be broken up into separate ganglia, which are connected with one another by longitudinal commissures. Each gan- glion, again, is broken up more or less regularly into two halves, which are connected together by transverse commissures. The two ventral chords then form a chain of ventral ganglia (Fig. 64.). In many Hirudinea the longitudinal chords of the ventral medulla are, during the early stages, sepa- rated fro
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